A membrane which is selectively permeable to specific components in a solution or a liquid mixture, such as an emulsion or a suspension, is generally called a selective permeable membrane. In the past, membrane separation treatment utilizing such a property has been carried out for only aqueous liquid mixtures. As is well known, permeable membranes composed of celluloses such as cellulose xanthate, cellulose acetate or cellulose nitrate, etc., as a starting material are not generally resistant to organic solvents, and permeable membranes prepared from synthetic resins such as polyvinyl chloride, polypropylene or polystyrene, etc., dissolve in organic solvents or are swelled by the organic solvents if they do not dissolve therein, as a result, they cannot be used as a permeable membrane.
However, membrane separation treatment must be capable of being used in treating not only aqueous liquid mixtures but also organic liquid mixtures and particularly organic solutions. Accordingly, development of selective permeable membranes for such purpose has been desired.
In order to satisfy this requirement, it has been already proposed to produce a selective permeable membrane composed of a polyimide prepared from 1,2,3,4-butanetetracarboxylic acid or derivatives thereof and diamine, as a starting material (e.g., as disclosed in U.S. Pat. No. 4,240,914, corresponding to Japanese Patent Application (OPI) 71785/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")). The present inventors have succeeded in developing a permeable membrane suitable for ultrafiltration of, particularly, organic liquid mixtures by using the above-described polyimide as a starting material, as disclosed in U.S. Patent Application Ser. No. 151,061, filed May 19, 1980, corresponding to Japanese Patent Application 61359/79. However, this ultrafiltration membrane does not separate very well solutes having a low molecular weight as low as 2,000 to 10,000, because the molecular weight cut off of the membrane is more than 20,000.